The progressive development of the semiconductor industry towards smaller and smaller structural widths with growing wafer diameters leads to constant adjustments of the machine parks in the semiconductor factories.
Even when a wafer size is kept, machines (tools) in a semiconductor chip production line are replaced over and over again due to technical progress during the life cycle of a structural width. These new tools then mostly improve the production speed or reliability.
Modern chip production lines have throughput times of several months in part for a wafer. At the same time, there is a requirement for processing to be individually controllable right down to individual wafers. Furthermore, some wafers are to be treated in a preferential manner, whereas others have no particular urgency. These priorities can change. The wafer production (processing) is to be documented in a detailed fashion throughout the entire run so that the requirements of quality assurance can be fulfilled. In addition to the pure processing of the wafers, however, the conditions under which the wafers are stored between the individual processing steps are of central importance. Chemical and physical changes also take place in a quiescent state, especially to the wafer surface, depending on the environmental conditions. This leads to a situation in which the storage conditions are to also be defined and monitored.
A series of technical solutions have been developed to meet these diverse and partially divergent requirements.
In modern semiconductor production (chip production) factories, the wafers are put together into units (lots—usually 25 wafers) and moved in transport containers. Although these containers were frequently manually transported in older semiconductor factories (fabs) and the containers were simple plastic housings with drawers for the wafers, these designs no longer meet the current requirements. In the state of the art, the wafers are therefore transported and
stored in FOUPs (Front Opening Unified Pods). Drawers to hold the wafers are likewise found in these FOUPs. Moreover, however, a FOUP can be transported by machine. The front door is also opened and closed, and the wafers are removed and inserted, by machine.
Using the FOUPs, fully automated production lines are realized in which the FOUPs are automatically transported from one tool to the next, or from a tool to an intermediate storage area, or from that to a further tool. Defined storage conditions are to be created in the intermediate storage area that prevent, in particular, storage durations of different lengths from leading to different surface characteristics of the wafers. Purging the surfaces of the wafers with inert gases has therefore proven to be advantageous to push oxygen out of the FOUPs and thus prevent oxidation and diffusion processes or to at least to strongly limit them. Argon or nitrogen is used as a suitable purging gas. The locations of the FOUPs in intermediate storage areas that are attached to the ceiling of the clean room are called FOUP nests. FOUP nests are understood below to mean all of the locations of FOUPs that serve to provide intermediate storage or to purge the wafers with purging gases without being part of a tool. Thus, FOUP nests do not necessarily have to hang from the ceiling of the clean room.
The FOUPs are transported by their own transport devices. They are usually realized in the form of rail systems on which the FOUP grippers move. An example is described in US 2010/0228378 A1. The rail system is designed as OHT (overhead hoist transport). The rails are arranged here above the tools and the traffic paths in the clean room. Technical solutions are known, however, that provide for transport systems installed on the floor of the clean room. The FOUPs are raised and lowered via cable-borne FOUP grippers in US 2010/0228378 A1.
The FOUPs are unloaded in the load ports of the tools, the wafers are fed into the processing and, after that, the FOUPs are loaded again. The wafers are not processed in the FOUP nests of the intermediate storage area or areas, but are instead merely flushed with inert gas. This flushing is called purging. The intermediate storage areas are frequently designed to be FOUP nests hanging down from the ceiling of the clean room on struts. When the FOUPs are put down
in the FOUP nests, they are set on base plates that have guide pins (kinematic coupling pins) and gas connections. These gas connections engage in a positive-locking and sealing manner with corresponding counterparts in the floors of the FOUPs. The supply of gas is made possible in that way. The gas supply system can likewise be installed overhead or, because clean—room floors are usually perforated double floors, in the intermediate area between the perforated floor and the lower floor of the clean room.
The designs of the FOUPs, their base plates and the FOUP nests vary depending on the manufacturer.
In the state of the art, the purging process is frequently controlled via a central computer that, as a drawback, can only address groups of FOUP nests in a number of embodiments. The gas flow is enabled even when not all of the FOUP nests of an intermediate storage area are occupied, which can lead to an escape of inert gas (which is harmless in technical or health-related terms due to the very high rate of air exchange in the clean room). The gas consumption increases significantly because of that, however.
As explained, the tools of the semiconductor industry are frequently replaced in the clean rooms. These machines are usually special designs with substantial dimensions, so transport in or out is problematic. In particular, it is not only necessary to disconnect all of the resource lines (electrical energy, possibly water, various gases); the corresponding supply systems have to likewise be removed. This also applies in an analogous fashion to the FOUP transport systems or the FOUP intermediate storage areas. They have to likewise frequently be removed to create working space. The situation intensifies when transport systems installed close to the floor are involved. The newly installed tools also include FOUP nests under certain circumstances that do not match the ones previously used in terms of the positions of the connections of the base plates.